Roland Tollenaar wrote:
> Hello,
>
> I have three questions I would like to pose here.
>
> 1-Is it possible to subdivide a task to control at what time a certain
> command is executed within the task? Example: I have a 1ms task. I send
> a CAN sync message at the beginning of the task but I would like to wait
> about 300us before performing a read to ensure that all the messages
> emitted as a result of this sync have come in. Now I will have an odd
> 700us to use the data that has come in to calculate outputs. However I
> want the output to be put out as close as possible to the end of the
> task time. Is there an elegant manner to delay the read and the write to
> give me this kind of control within a taks. If not any suggestions as to
> how I should tackle this.
The task timing services allow to suspend and then resume the task at a
defined time, either by specifying a releative delay or an absolute
time, e.g. rt_task_sleep() and rt_task_sleep_until() for the native skin.
>
> 2-For development I am still using the PEAK CAN dongle. We have
> establisched that the ISR responding to an incoming message interrupt
> takes 200us to complete. Is anything known about how long it takes
> before a message that has been sent is put on the bus? How does this
> happen? Reason I ask is to know what kind of variance I can expect in
> time between the moment I send the output values and the moment it is on
> the bus. If there are routines between those to points that allow
> themselves to be pre-empted or for some other reason have varying
> execution times my control performance will be badly degraded.
Reading, but also writing a byte to the port register task time. I
think, that setting up a message for transmit takes as much time as
reading it. Writing is also done with interrupts disabled.
> 3-How do I interpret the relative time stamps that can be made visible
> with rtcanrev ? IS the time indicated the time between each message and
> the last time the message was posted? Or between the message and its
> direct predecessor?
The latter one if the option "-R" was used:
$ rtcanrecv -h
...
-T, --timestamp with absolute timestamp
-R, --timestamp-rel with relative timestamp
Note that the timestamp is added in the ISR.
Wolfgang
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